The study of bacterial chemotaxis offers an approach to understanding the basic fundamentals of the biological signalling systems at the ultimate molecular level. As more is learned about sensory-signalling systems of higher eucaryotes such as the visual system or the hormone receptor systems, it has become obvious that bacterial chemotaxis is not a model for these phenomena but rather another example of the same basic biochemical mechanism. Because of the similarity of all of the signalling systems, understanding of an aspect of one system has immediately been applicable to the others. For example, the GTPase activity of the transducin protein of the visual system is now known to be analogous to the GTPase activity of the G protein in the beta-adrenergic hormone system. Covalent modification of the signaller by methylation in bacteria is analogous to phosphorylation of the signaller in the rhodopsin system. One of the advantages of the bacterial system is the wealth of genetic data accumulated over the years. There are greater than 40 genes and 12 operons required for a fully functioning chemotaxis system in E.coli. It is this availability of genetics which allows the bacterial system to gain insight not possible with the eucaryotic systems. For example, the genetics has been used to identify all of the gene products involved in bacterial sensory reception and signal transduction. Therefore, all of the parts of the system are known. On the other hand, some aspects of biochemical analysis are more advanced in the higher eucaryotic systems. This has been true because sensory organs, such as bovine eyes, are readily available in large quantity. It has not, however, been possible in the past to isolate the sensory organelle from bacteria. Now, by exploiting in vitro genetic recombination, it has been possible to overproduce and purify milligram quantities of individual chemotaxis proteins. this ability coupled with the well-developed genetics and the biochemical precedents developed in higher systems will enable the sensory system of bacertial chemotaxis to be understood at the molecular level.